System for the translation of intelligence



Aug 11, 1959 R. E. STURM ET AL 2,899,494

SYSTEM FOR THE TRANSLATION oF INTELLIGENCE AT Low sIcNAL-To-NoIsE RATIosFiled June 2, 1954 5 sheets-sheet 1 llg- 11, 1959 R. E. sTURM ET Al.2,899,494

sYsTEM EOE THE TRANSLATION 0E INTELLIGENCE AT LOW SIGNAL-TO-NOISE RATIOS5 Sheets-Sheet 2 INVENTORS BY MMM/.7W

Filed June 2. 1954 ATTORNEY Augl1, 1959 R. E. sTURM ETAL SYSTEM FOR THETRANSLATION OF INTELLIGENCE AT LOW SIGNAL-TO-NOISE RATIOS 5 Sheets-Sheet3 Filed June 2, 1954 I N VENTORS ATTORNEY United States Patent O SYSTEMFOR TI-m TRANSLATION OF INTELLI- GENCE AT LOW SIGNAL-TO-NOISE RATIOSRalph E. Sturm, Pikesville, and Russell H. Morgan, Baltimore, Md.,assiguors to Bendix Aviation Corporation, Baltimore, Md., a corporationof Delaware Application .lune 2, 1954, Serial No. 433,955

'5 Claims. (Cl. 178-6.8)

This application relates to the translation of intelligence having a lowsignal-to-noise ratio, and more particularly to the augmentation of alow level signal without emphasizing noise. The invention has itsprimary application in the eld of fluoroscopic screen intensification,but as will appear more fully hereinafter, it is by no means limited tothis lield.

The use of X-ray and fluoroscopy to examine patients produces an imageon the fluoroscopic screen at such a low light level that the examineris required to darkv adapt prior to the examination in order to see theimage. It is highly desirable to brightensthis image by an order of athousand to ten thousand times in some manner in order to eliminate thenecessity for dark adaptation and thereby bring the light level up towhere the acuity of the eye is very much better that it is at the lowlevels ordinarily found in this practice.

' Shortly after X-rays were discovered, it became known that theyproduce ionization effects on materials through which they pass, and, inparticular, they produce deleterious effects on live tissue if used insuicient quantity. Therefore, limits for X-radiation employed indiagnostic Work have been adopted as standard practice. Since X- rayspossess an accumulative elfect on tissue, the level of radiation whichis incident on the body must be specified for the different proceduresin diagnosis. With a satisfactory intensifier it would be desirable, ifpossible, to lower the presently established limits of radiation to thelowest value at which an acceptable observation might be made.

In normal iluoroscopic work Without the aid of an intensifying system,standard practice is to use an X-ray tube at a distance of approximately18 inches from the human body, exciting this tube with an electricalpotential consistent with the thickness ofanatomical structure beingobserved (which potential generally falls into the range of 40 to 100kilovolts) and using a tube current of approximately 5 milliamperes.These conditions usually produce a safe X-ray dosage during a normalfluoroscopic examination.

In lluoroscopy of the chest of a medium size human adult at a tubecurrent of 5 milliamperes, the light produced by the uoroscopic screenis of the order of -2 milliliamberts. If one wishes to observe theantero-posterior abdominal region (that is, fore and aft) the increasedthickness of the structure decreases the light obtainable from thefluorescent screen to approximately one-tenth of that obtained from thechest, or l0"3 millilamberts, and examination of the abdomen in thelateral direction (that is, perpendicular to the fore and aft) decreasesthe light by another factor of about one-tenth, producing only 10-4millilamberts. Because of these low light levels iluoroscopy isfrequently quite unsatisfactory, and the diagnostic information whichmay be obtained is severly limited.

Accordingly, attempts have been made in the prior art to utilize screenintensification and thereby eliminate the serious deficiencies ofuoroscopy at low light levels.

observations of the abdomen either in the antero-pos terior or lateralposition have been unsatisfactory be ice Various intensifierarrangements have been employed.

One of these, a closed link television system including av pick-up tubefocused on the iluoroscopic screen, would appear to lend itselfparticularly well to the concept ofA screen intensification. Tests ofsuch a system have pro` duced good usable observations of the chest.However,

cause of fluctuations in the picture caused by the random noise of thesystem. It has become increasingly evident that the conventionaltelevision intensifier is limited primarily by the noise level of thesystem. Use of the best components available has failed to produce anysigni' cant improvement in the performance of the conventionaltelevision intensifier.

It has been generally assumed in the prior art that the; noise observedon the viewing screen is the manifesta tion of random noise existing innature, such as orthicon beam noise, shot effect, thermal agitation orresistor noise,l

etc., and that since such noise is inherent in the system, its effectscannot be eliminated. Contrary to this generally accepted view, we havefound that the noise present on the television screen is far in excessof that which would be predicted from classical noise theory, andmoreover, instead of being random, such noise has a definite' spectralcharacteristic, which produces much greater de.

' terioration of the picture than would be expected from theory. Morespecically, we have discovered thatthe excessive noise present on theviewing Yscreen is causedby shock excitation of underdamped modes ofvibra'- tion of the circuitry by noise occurring in the input, whichresults in the augmentation in amplitude and compression into 'a narrowfrequency band of the random noise exist-` ing in nature. The presentspecilication teaches a method and system by which these phenomena maybe substantially eliminated. In consequence, we are able t0 produce fargreater screen intensification than has ever been produced before.

Accordingly, it is an object of the invention toprovide a system for andmethod of translating low levelv signals without emphasizing noise.

Another objectof the invention is to provide a meth,- od of and systemfor amplifying intelligence having a' low signal-to-noise ratio.

Still another object of the invention is to provide an improved methodof and system for intensifying an image on a fluoroscopic screen.

A further object of the invention is to provide a novel amplifiersystem.

An additional object of the invention is to provide a novel scheme forintroducing blanking signals into a tele-v` vision system or the like.

A still further object of the invention is to provide a novel method ofand system for operating a cathode follower or the like.

These and other objects of the invention will becomeIr ing blankingsignals into the intensier;

Figure 4 is a graphic illustration of the operation of a conventionalcathode follower under conditions to be described hereinafter; l y

Figure 5 is a circuit diagram of a modified arrangev f l ment forintroducing blanking signals; and

Figure 6 is a block diagram showing the arrangement of the amplifiersections of the invention.

Figure l illustrates the general scheme of the invention. In thisembodiment the invention has been applied aitX-ray intensificationVsystem which includes an X- rayV control 10,for *controlling andoperating an X-ray tnhe 12,1which projects a beam of X-rays onto a. uo-@scent-'screen 16 through a subject 14. A grid 15VV may beV placedbefore fluorescent screen l16 to reduce scatter.` Theimage produced onthe uorescent screen 16 under the; action of the X-rays is focused by anoptical system represented by lens 18'onto the light sensitive elementof a pick-up tube 20. This tube may bean image orthicon ofthe typeconventionally employed in television prac,- tice;l Block 20 may alsoinclude the necessary sweep cuits and controls for the` image orthicontube.

"The electrical signals corresponding to the image produced on thelight'se'nsitive element of the pick-up tube are` applied to acritically damped amplifier 22 which will be described in more detailhereinafter. Block 22 may include controls to set the contrast of thepicture produced on the fluorescent screen of a kinescope 24, to whichYis applied the amplified signals from the critically damped amplifier2,2. A pulse former and Shaper 26 supplies the necessary pulses toinitiate the operation of the sweep circuits in the image orthiconsystem andthe kinescope system at precisely the same time so that thepicture which is broken up into small increments by the, irnage orthiconwill be re-assembled into exactly the same increments at a greaterbrightness by the kinescope 24. If desired, the screen of the kinescope24 may be photographed toprovide a permanent record, of theobservation'. 'A suitable power supply (not shown) furnishes all` of thepower requirements lfor the intensifying unit. As will appear below, theentire system is at least critically damped, Le., thatpart of the systemthrough which. the video signal passes.

In ordinary television practice, Le., aV 525 lineA interlaced scanningsystem, amplifiers must be capable of.f

maintaining good amplitude and pliaseresponse over a frequency range ofthe order of 60 cycles per second up, to` approximately 4 mcgacycles persecond. vIn the system of Figure lY this range may be extended to.approxi-- mately 8 megacycles in order'to, be sure of, obtaining the.lbest resolution possible, Generally, the useful frequency range may befromv about 5,0 cycles to 15 mcgacycles per second.

It is well known in the art that ordinary amplifier tubes in,resistance-capacitance coupling will not cover the frequency rangerequired in television practice while producing optimum gain withouttheinclusion of special peaking circuits which compensate for the input'capacitance of the tubes as well as the capacitancejassociated with thelayout, wiring, and the components Both shuntV and series peaking aswell as a combination of the two are employed. Many analyses of suchcircuits have been published. For reference, twopapers maybe mentioned,one by McLachlan, published in the Philosophical Maga.- zine, volume 22,1936, page 481, entitled The Requirements of an Amplifier in Order toExtend its Range; and another by Bedford and Fredendall, published inthe Proceedings of the IRE, volume 27, 1939, page 2,77, and. entitledTransient Response of` Multistage Video Erequency Amplifiers. The latterarticle attempts tor set forth the requirements of such amplifiers,using Fourier series to predict the response of a multistage `amplifierto a unit voltagesignal (sometimes called the Heaviside signal).

As` indicated above, it` is standard practice toemploy peaking inamplifiersto compensate for the input-capacitance` of the tubes, wiringcapacitance, etc. Peaking is usually accomplished` by-adding theIrequired amount of nductance to correct for amplitude and phase`angle-dis,` tprtion. In general, peaking renders the circuit oscillatoryin one or more modes of vibratiomy andoverslioot,

.M "nascar-194.y

and subsequent oscillation occur because of the oscillatory condition.In order to adequately correct for phase and amplitude distortionwithout employing an excessive number of tubes, which in turn wouldincrease the noise of the system, it is necessary that the conventionalcircuits be oscillatory. Even though the circuits may have avery low Q,if a step-function with a sufhciently fast rise time is applied thereto,oscillations at the natural fre-- quency of the circuit will occur, andif' before the latter have completely died down, another step-functionis applied, the amplitude of oscillations in general will build up. If aseries of such step-functions is applied, in quick V enough succession,the oscillations will build up.

In conventional practice, an amplifier of many stages connected incascade is used; each stage possesses the above oscillatorycharacteristic, and each stage oscillates at very nearly the samefrequency. Consequently, once the applied step-functions produceoscillations in the first stage, this stage acts as a force functiondriving voltage for the next stage, andthe latter in turn acts as aforce function driving voltage for the stage following it. While inpractice, each stage does not operate` exactly at the same frequency,because of variations in wiring capaci'- tance, stagger tuning, etc., ingeneral the oscillating frequencies are close enough together that eachstage can be considered a force driving function for the following. Itcan, therefore, be said that the oscillations built up in the firststage of a conventional amplifier will be largely enhanced by thesuccessive stages.

Random noise is transient in that no interval of periodicity exists. Wehave discovered that such noise acts on oscillatory circuits (as foundin conventional amplihers) in a mannersomewhat similar to that of thevarying square waves asV indicated above. The amplitude of the:fundamental oscillating frequency of the respective vcircuits builds upandtherefore accentuates the noise in addition toproducing a;periodicity which we have foundl to be especially deleterious totelevision picture quality.` From the noise equation E2==4KTR(Af), whereE=voltage, K=Planks constant, T=absolute temperature, R=

resistance in ohms, Vand Af=band width in cycles per sec-- ond, it isevident that there is as much` noise power available between onemegacycle and two mcgacycles as there` is in the, spectrumbetween twoand three mcgacycles, etc. In other words the power distribution ofnoise signals, is equal in any given frequency interval of thespectrunn` It can, therefore, be seen then that if one applies aseries.` of .square waves re-occurring in a random nature inf thefrequency spectrum from 60 cycles to 8 mcgacycles to` anoscillatoy-.circuib and in. particular, if the circuits: fundamentaloscillation occurs at approximately 2 mega-f cycles,A everything fromabout '2; mcgacycles to 8 megacycles4 will-tend to build. up an.amplitude in a narrow interval abouty mcgacycles The'random pulsesoccurring between` 2 and 8 mcgacycles will tend to be compressed into anarrow band at 2 mcgacycles,v and. consequently the amplitude at thisfrequency will rise. Thus, random noise when applied to a` conventionalpeaked amplifier will bev accentuated inA amplitude and com.- pressedinto anarrow: spectral band. While the fore:` going analysisV producesresults` which appear. to agree. with experimentall observations, thereare many other methods of analysis which could be employed, and it is`not essential that this simplified analysis be used to expresstheconditions which, exist in this. type of circuit,

An amplifier isan active, not a passive network. The: standard. amplierhaving shunt peaking, series peaking or af combinationrof both` may besimplified into a passive element comprising, the-peaking. circuittogether with other. passive.components,. and anA active element com.-prising the. vacuumtubes andassociatedequipment .which act as generatorsfor Vthe passive elements. Itmay be shown. that. a passive filter. witha. flat bandv pass. frequency response and= linear; phase characteristicwill shockexcite atfits mid-baud frequency when. applied sighals changeabruptly, even when such signalsy are entirely outside the pass band.(See pages 477 through 486, vol. II of Communication Networks, byGuillemin, published by John Wiley, 1935.) Furthermore, the filter isnot required to have circuit parameters normally considered oscillatory.To minimize this effect, the capacitances and inductances of the circuitmust be dealt with in a specific manner, for example, in the manner ofthe line amplifier of the invention as set forth below.

We have discovered that the solution to the problems set forth above,that is, the elimination of circuit oscillation in response to randomnoise excitation and the accompanying amplitude accentuation andfrequnecy compression of random noise, lies in the use of circuits allof whose vibration modes within the entire operating range offrequencies are at least critically damped. If the standard peakedamplifier circuit were modified so that the gain of each stage were lowenough to prevent oscillation, many additional stages would be requiredto produce the necessary over-all gain, and ultimately the noiseintroduced by the input tubes and their parameters would defeat thepurpose. The critically damped amplifier which forms a sub-combinationof the present invention utilizes the long line or distributed constantprinciple. This general principle of amplifier design is, of course, notnew to the art, since at the higher frequencies where the ordinary shuntor series peaking is not effective in correcting phase and amplitudedistortion, amplifiers have been built on the theory that each tube is apart of the distributed capacitance of a long line. Such amplifiersoperate satisfactorily up to frequencies of several hundred megacycleswhen not limited by circuit effects outside the tubes. They have beenemployed to obtain Wide band-Width and high gain, but it is to be notedthat no reference is found in the prior art to the adaptation of a lineamplifier to prevent the emphasis of noise in the translation ofintelligence having a low signal-tonoise ratio. In fact the large numberof tubes required by such an amplifier would lead one to believe thatline amplifiers are unsuitable for such use, because of the increasednoise which would be expected from the employment of such a large numberof tubes.

Figure 2 illustrates one section of the amplifier of the invention,Actually the complete amplifier may comprise several sections similar tothat illustrated, each section connected to the previous one in cascade,as shown in Figure 6. Each section comprises a plurality of drivingdevices exemplified by the tubes 28 to 4f). In this particularembodiment, seven pentodes, such as the 6CB6, may be employed. Thecontrol grids of the respective tubes are connected to a grid line 46comprising -a series of coils 48 to 62 and condensers 107 to 120.Successive coils may be wound in opposition to reduce the mutualcoupling between adjacent coils to the lowest level possible, but thisis not essential. The grid line is terminated at its respective ends inits surge impedance by resistors 64, 66, respectively, and the series ofsmall padding condensers 108 to 120 are employed to correct forvariations in tube capacitances and to bring the surge impedance of eachsection of the line to the correct value. By proper adjustment the gridline may be made substantially reflectionless.

The anodes of the respective tubes are connected to a plate line 68comprising coils 70 to 84, which also may be Wound successively inopposition. Here again a plurality of padding condensers 94 to 106 areemployed to adjust the respective sections of the line to the correctsurge impedance. The plate line may be terminated at one end by aplurality of resistors 86, 88, 90. The other end need not be terminatedin its characteristic impedance, and this arrangement substantiallydoubles the gain, as is known in the art. As will become more evidenthereinafter, reections produced by failure to terminate one end of theplate line in its surge impedance will not greatly affect the operationof the circuit Where pentodes are employed, because of the fact thatbeyond a certain voltage the plate voltage of a pentode does notsubstantially determine its plate current. The terminating resistors 86,88, 90 on the plate line may be quite critical, since in thisapplication they are required to have about 13 watts dissipation withnegligible inductance. Ordinary non-inductive resistors of the wireWound type may not be satisfactory but the type R33 non-inductiveresistors produced by the Corning Glass Company, or its equivalent, maybe employed satisfactorily.

An input driving device illuustrated by pentode 42, which may be a 6AH6tube, has its anode connected to inductance 48 of the grid line and itscontrol grid connected to input terminal 156 through a coupling networkincluding coupling condenser 158 and grid return resistor 160. Asuitable cathode load resistor 138 is provided. It will be noted thatresistors 136 and 138 in series form a cathode load for the line tubes28 to 40. These resistors are connected to the respective cathodes ofthe line tubes through lead 134 and are by-passed to ground throughcondensers 146, 142. The flow of plate current of the line tubes throughresistors 136, 138 produces a small positive feed back which results inbetter low frequency response. The feed back is operative only at theextremely low frequencies, since the higher frequency signals areshunted by condensers 140, 142; control over the feed back isaccordingly obtained by selection of the values of condensers 146, 142.Cathode load resistor 138 may be shunted by a small condenser (notshown) to provide high frequency peaking and phase shift, if desired.

The passage of the line tube plate currents through resistors 136, 138is also utilized to provide well regulated voltages for the grid line 46and to decouple the grid line from the power supply. In operation, eachline tube may have approximately 12 milliamperes flowing through it, and7 line tubes will give a total current of approximately 84 milliamperes.The resistance of resistor 136 may be approximately a thousand ohms, andthe resistance of resistor 138 may have a relatively low value. Thecombined line tube plate currents passing through these resistors inseries produces a regulated potential of approximately 84 volts at thecathodes of the line tubes, and this potential is applied to the plateof input tube 42 through terminating resistors 64, 66. Condensers 140,142 provide a filter for the input tube plate potential.

The screen grid of tube 42 is fed from the B supply at terminal 122through variable resistor 146 and fixed resistor 148, and is by-passedto ground by condensers 152, 154-. Resistor 146 may be employed tocontrol the plate current of tube 42. Since the D.C. plate current ofthe input tube flows through resistors 64, 66, which lie in the controlgrid to cathode path of tubes 28 to 40, resistor 146 may be employed tocontrol the grid bias on tubes 28 to 40.

An output translating device, which has been illustrated as a triodetube 44 connected as a cathode followeris coupled to that end of theplate line which is not terminated in its characteristic impedance, by aphase corrective network 171 which may comprise variable inductance 170,capacitor 174 and resistances 172, 176. A coupling condenser 166, a gridreturn resistor 168 and a cathode load resistor 164 are provided for theoutput tube. The output terminal 92 is connected to the cathode of thetube.

Where large condensers, such as electrolytics, are required in thecircuit illustrated, they must be shunted by smaller condensers in orderto ensure the desired high frequency response. lt is Wel-l known that anelectrolytic capacitor, for instance one having a capacitance of ahundred microfarads is not satisfactory for use at high frequencies. Toovercome this each of the large condensers is shunted with a smallercondenser, such as a .0l microfarad. Thus in Figure 2 condensers 130,140 and k154, which may be large electrolytic capacitors, are shunted.by smaller condensers 1,32, 142 and 152 respectively.r

'IheB supply voltage ted to eaehot Hthe amplitiertubes from :terminal12.2 should rbe very lcarefully regulated- Since .shoeklexeitation' aswell .as standing Waves et high frequencies `mavoeelrr ,ou thelead Wiresfrom the .Bzsupulnladeeouplius network Consisting of e resistor -124 andeeudeuserlo is inserted to deeouple the amplifier from the power supplyand toprevent these effects. The value of resistor 1.24 Ais mede largeenough so that the induetauee 'of the line A.feeding the amplifiertogether .with the distributed capacitance will Vnot osellate when shookexcited.

The -screen grids of tubes 28 to 40 are fed from the B supply throughdropping resistor 128 and condensers 130,I 182, whichforma lter network.A plurality of resistors 1.340 tlll'ollgh 146a are inserted in serieswith each of the ,screen vgrids of the line tubes. These resistors areempioyed to prevent spurious oscillations due to the inductance andcapacitance of Vthe lines feeding the screen grids of the particulartubes, that is, they are employed to ensure at least critical damping.In practice, it may be necessary to insert small resistors (such asresistors 161, `1152, 165 associated with tubes 42, 44) in series withthe grids and plates of all tubes except the line tubes per se tocounteract any tendency toward oscillation of the in.

ductance of the leads taken in Yconjunction with the distributed andtube capacitances. In this connection it should be noted that foroptimum results it may, in some instances, be necessary to insert smallresistors in the filanent kleads of the tubes, in the lines between theamplifier sections, and also in the circuits represented by Vblocks and24 vin Figure 1, including the dynode circuits of the iniage Vorthicon.Damping resistors are inserted wherever an oscillator-'y condition ofinductance and capacitance would exist in their absence. These resistorsmay be of sonic convenient value, Vsuch as of the order of 47 ohms. Toensure the lowest possible noise level in the final kinescope indicator,it is essential that all possible modes of vibration of the circuitsYfrom input to output be at least critically damped. At the frequenciesfor which the amplifier is employed, even very short cathode leads, for'example, may have sufficient inductance to cause oscillations and otherspurious effects. This is particularly true of the cathode circuits,because any inductance here will be very greatly enhanced by theamplification of the tube. ,It was found in one embodiment that anundamped lead length of 4more than half an inch on the cathode of theinput tube 42 was suflcient to allow spurious oscillations and degradethe picture on the kinescope because of an apparent enhancement of therandom noise.

Considering the operation of the circuit illustrated in Figure 2, a lowlevel signal at input terminal 156 is applied to the control grid ofinput tube 42 through the coupling network 1 58, 160. For purposes ofillustration itis assumed `that the input signal is a square wave withpositive polarity. This wave will increase the current in the inputtube, which will, by means of resistors ,64, 66, produce a decrease inthe voltage at the plate Vof tube 42. For extremely low frequencies, theinductauces 48 through 62 have practically no effect, and resistors 64,66, are essentially in parallel. However, at high frequencies, theseinductances do have substantial effects, and consequently, it can beseen that the input signal produces a negative square wave whichproceeds down the grid line 46 toward resistors V64, 66. It is evidentfrom line theory that a line has a finite propagation time dependingupon its parameters, that is, it takes a tinite length of time for avoltage wave to move down the line. The propagation time may becontrolled by ,the parameters R, C and L including parallel conductances(not shown) in each section of the line. The padding condensers 108through 120 may be adjusted to .compensate for varying input capacitanceof the tubes so that the propagation constant is the same for eachsectionlof the line. The propagation constant of the line 3S e WholeWill, therefore, be linear and the negative signal yat the input of,thelinewill Vmove smoothly and linearly toward resistors 64, 66. A wavewhich is incident upon either Aof resistors 6.4, 66 .will be completelyabsorbed, r`since the -line is terminated at each end in its surgeimpedance.

11i-the plate line;68; the padding condensers v94 through 106maysimilarly be adjusted to ensure a linear propagation yconstant vfor theplate line Vas a whole. While the termination comprising resistors 86,88, of the plate line rnay -have a different value from the terminatingresistors of the grid line, the propagation constants for the two linesmay nevertheless `be made exactly the same. Assuming this lto be thecase, when the negative square wave producedat the plate of input tube42 reaches the control grid `of tube 28, it produces in the platecircuit of this tube a positive pulse which is an amplified inversion ofthe pulse incident upon the control grid. Current through tube 28, aswell as plate current for all of the other line tubes, must flowlthrough the termination 86, 88, 90. Thus, the pulse produced at theplate of tube 28 will start down the plate line toward its ends. Sincethe propagation constants of the plate and grid lines areidentical, asthe negative pulse moves down the grid line, the positive pulse willrnove down the plate line, and each time the negative pulse is incidenton the grid of a line tube, the plate of vsuch tube will add a positivepulse to the one already existing from the previous tube. The signalwill lbe built up as though all seven tubes had been connected inparallel and all of their trans conductances had been operating on theload resistance, 86, 88, 9,0.

,A grid line signal is `completely absorbed in `the ter minatingresistors 64, 66. However, in the plate line, since the far end is notterminated in its characteristic impedance, the signal will be reflecteddepending on the type of termination. Consequently, a reflected signalwill Vstart back down the plate line passing each of the tubes in turnand finally arriving at the terminating resistance 86, 88, 90. Here thereflected signal will be completely absorbed, because the line at thispoint is terminated in its surge impedance. As indicated previously, thereflected signal will not affect the operation of the circuit, becausethe plate current of a pentode is substantially independent of its platepotential beyond a certain potential.

The useful signal on the plate line passes through the phase correctivenetwork 171 and is applied to the control grid of cathode follower 44.The signal incident on the grid of the cathode follower produces asignal at the cathode which can be fed to the next amplifier sectionfrom a substantially low impedance source. This tube acts las animpedance changer and a decoupling tube, so that whatever is connectedto the output of the amplifier section will not have a substantialeffect on the characteristics of the plate line for this particularsection.

Three sections identical to that illustrated in Figure 2, with theexception of special input and output connections to `facilitateintroduction of blanking signals, etc. may be connected in tandem asshown in Fig. 6, producing a maximum gain of from 400,000 to 1,000,000.Such an amplifier system has been tested in a closed link televisionchain of the type illustrated in Figure l employing a standard imageorthicon tube of the 5820 type and it has been found that with criticaldamping the noise is reduced `by a factor of the order of 20 to 80 timesover the system using a standard shunt-series peaked amplifier. Goodpictures were obtained on the screen of the kinescopedown to lightlevels of 10'4 millilamberts, and in particular it was observed at thislevel that the small amount of noise that remained was entirely randomand did not interfere with the resolution of the picture nearly as muchas d id the previous periodic noise at the higher light levels.

The fact that the amplifier of the invention, `with its criticallydamped characteristics, does not accentuate noise also implies, and itis proved in actual practice, that any signal, and in particular of thesquare wave variety where the rise time is extremely fast, will bereproduced essentially faithfully, without overshoot or ringing, whereasin conventional amplifiers, such as the shunt and series peaked type,this is not the case. This means that much better resolution may beobtained throughout in a picture presented on the kinescope regardlessof the source of the signal.

The output of the final amplifier section is required to drive akinescope as indicated in Figure 1. Good design requires that theamplifier be able to drive the kinescope from cut-olf to cut-olf eventhough this may not actually be done in practice. For the type ofkinescope employed in the present system at least a 30 volt signal wouldbe required to accomplish this. In driving the nal cathode followerthrough a full 30 volts it was noted initially that the low frequencieswere handled very well with little or no distortion; however, the higherfrequency signals which were impressed on the input were notablydistorted. When square waves with extremely short rise time were fedthrough the circuit there was a noticeable curving off as the squareportion of the curve rose, that is, high frequency cut-off or highfrequency attenuation was noted. It was found that the capacitancebetween the cathode and the filament of the cathode follower as well asthe capacitance of the associated parts of the circuits delayed the risetime or fall time of the signal applied to the cathode so that it didnot follow the grid instantaneously. Under ordinary conditions such aphase lag could be corrected in the circuit if that were the onlyeffect. However, the Ilag of the cathode with respect to the grid causesthe grid 'to draw current, upsetting all of the relationships in thecircuit and consequently causing a badly deformed Wave in the output. Wehave discovered that this effect may be overcome by arranging conditionsso that the tube has a quiescent bias between cathode and grid which isalways equal to or larger than the signal applied to the grid. It hadbeen assumed prior to our discovery that conventional cathode followerswould handle signals up to frequencies at which transit time effectsbecome important.

Figure 4 illustrates the phenomenon discussed above. It can be seen thatwhen a square wave is applied to the grid of the cathode follower, thecathode voltage does not rise at the same rate and at time t1, forexample, the grid may be positive relative to the cathode by better than25 volts. The conditions at time t2 indicate that the maximum positivegrid-cathode voltage may reach 50 volts in the example given. The resultis a badly distorted output signal. The quiescent grid-cathode voltagemust, therefore, be chosen so that the grid is at least 50 voltsnegative with respect to the cathode in order to eliminate thephenomenon discussed. This may be accomplished by choosing the tube,plate voltage and cathode load, so that the quiescent current throughthe cathode load is sufficient to bias the cathode at least 50 voltspositive with respect to the grid, for the example given. The graphmakes use of linear curves for simplicity, but in practice these curvesare exponential.

As shown in Figure 6, the amplifier comprises three sections, an inputsection, an intermediate section, and an output section. Each section isdirect-coupled, but from section to section resistance-capacitancecoupling is employed. This allows the convenient introduction ofblanking and shading signals, which are preferably not applied directlyto the line tube stages. Figure 3 illustrates a unique way ofintroducing the blanking signal. This signal is generally employed tocut olf the beam of the kinescope during the return trace of the cathoderay so that the latter does not interfere with the picture, and in theparticular system disclosed it is also utilized to set the D.C. blacklevel in association with the circuits that follow so that contrastcontrol is obtained in the nal picture. In the circuit of Figure 3 thevideo input signal at terminal 202 is applied through a networkcomprising coupling condenser and grid return Aresistor 182 to thecontrol grid of a triode 178A. The blank signal, which may be fed from alow impedance cathode follower source, is applied from terminal 204 tothe control grid of a triode 178B through a coupling network comprisingcondenser 184 and grid return resistor 186. As indicated in the drawing,tubes 178A and 178B may be constituted by two sections of a dual triodetube. The triodes are provided with a common cathode Iresistor 188 andare connected to a source of B supply v196 through a decoupling networkcomprising resistor 194 and condensers 198, 200. The latter condenser isof relatively small value and shunts the larger condenser 198 (which maybe electrolytic) for the higher frequencies, in the manner set forthpreviously. Resistor 192 is a plate dropping or load resistor for tube178B, while resistors 181, 183,190 are small resistors employed toeliminate parasitic oscillations and to ensure critical damping. Thevideo input signal on the control grid of tube 178A is coupled from thecathode of the latter to the cathode of tube 178B. So far as the signalapplied to the cathode of tube 178B is concerned, this tube operates asa grounded grid ampli- 'er, which allows operation at a higherfrequency, because the input capacitance is quite low. The control gridof tube 178B serves as a mixer grid to which the blank signal is fed,and the output is taken on lead 195 from the plate of this tube. Thedual triode 178AB may Vconstitute the output tube corresponding to tube44 of Fig. 2 for the intermediate amplifier section of Fig. 6. Thissubstitution is indicated in the drawings and may be accomplished bybreaking the circuit of Fig. 2 at points X and connecting in place oftube 44, etc., the circuit of Fig. 3 at the points X1. This arrangementoperates well up to and including frequencies of 15 megacycles, givinglan appreciable gain, depending on the transconductance of the tubes,Without employing peaking devices. Thus, an amplifying stage is providedwhich may be used together with the line arnplier to obtain additionalgain and to solve the problem of mixing without introducing anydeleterious eects on the signal, as would occur with a stage in whichpeaking were employed in order to proper-ly correct for amplitude andphase distortion. The Ause of a dual triode allows extremely shortcathode leads and, therefore, minimizes cathode inductance. If separatetubes were employed, the parameters of the cathode circuits could beadjusted to produce a ltering action, if desired.

Alternatively, blank signals could be introduced by employing a secondpentode in parallel with the input tube 42 in Figure 2, connecting theplates of the tubes together and utilizing separate screen grid, cathodeand control grid connections. This arrangement is illustrated in Fig. 5,wherein tube 42 and associated components correspond to those shown inFig. 2; only that portion of Fig. 2 necessary to the description isrepeated. The anode of parallel tube 210 is connected to the anode oftube 42, and the cathode is connected to ground through a bias networkincluding resistor 212 and condensers 214, 216. The screen grid of tube210 is fed from the B supply through a variable dropping resistor 224and by-pass condensers 226, 228. Conden'sers 216, 228 may be employed toshunt larger condensers 214, 226, as set forth previously. The blanksignal from terminal 222 is coupled to the control grid of tube 210through resistor-condenser network 218, 220. In operation, the bias oftube 210 is adjusted so as to prevent large shunting of tube 42, therebypreventing substantial loss in gain for tube 42. It has been found thatsatisfactory operation results if tube 42 carries 80% of the combinedplate current, provided the blank signals are sufficiently strong. Whilethis arrangement makes an excellent mixing system, there is some loss ofthe normal gain of tube 42.

The introduction of a shade signal, indicated in Fig. 6, may beconveniently accomplished by applying the re,-

quired saw .tooth voltage to )the cathode and/.or rthe'oorr trol grid ofthe ,input tube (corresponding to .tube 4 2 .in Eig- 2)ofetheinterrnediate amplifier section. The nood for such signals is.well knowniin ytelevision practico, .and systems -for .applyingsucbsignals .are also Well known- Contrast `control may be Vachieved .byinserting a gain control potentiometer in the input to ,the intermediateamplifier vand a suitable black level setterinthe output of theoutputamplifier section.

From the `equation fornoise referred to previously, ;i t is evidentlydesirable Ito limit the spectrum of 4the system in vorder to reduce thenoise t the lowest IJOSSblt Valli@- This maybe done by limiting both thetop and bottgrnof the signal pass Ibandof vthe system, preferably vatitsginput.

As indicated previouslwa system `may be .shock excited by a signal,suchas a spurious oscillation, whichis entirely outside the pass band of the system. It has been fOllnd that when one controls the phase andamplitude gharacteristics of the limits of the pass band so as toproduce the narrowest pass band that is necessary to transmit theintelligence and at `the same time -to prevent oscillations, the lowestpossible noise level is obtained.

The-term critically damped as employed in thespeification and claimsdescribes the condition of a circuit-in which the ability of the circuitto oscillate just ,ceases -to exist. vFor example, in .a simple seriescircuithaving inductance (L), capacitance (C), and resistance (R),critical damping .exists When the solution to thederential equation forthe current in the circuit is such ,that the discriminant is equal .tozero, or Where If the left-hand term of this Yequation is greater thanthe right-hand term, the .circuit is over-damped. In both instances thecircuit is non-oscillatory, but if the lefthand term is .smaller thantheright-hand term, the circuit is oscillatory. As employed in thespecification and claims the term at least critically damped refers .t0@circuit which is either critically damped or over-damped, i.e.,non-oscillatory.

The ideal condition of exact critical damping vis difficult to achieve,land in practice the condition is approached as a limit from the regionof over-damping. The amplifier must be at least critically dampedthrough its entire operating range, which includes its pass band .and'band skirts. Auxiliary circuits of the amplifier through which thesignal does not pass, such as power supply leads, leads for insertingblanking signals, etc., should ofcourse be at least critically dampedtoV prevent noise enhancement, but may be Isubstantially over-dampedwithout detracting from fidelity of reproduction.

It should be evident from the foregoing description of the inventionthat the use of a system which is at least critically damped for allmodes .of vibration within its operating frequency range eliminates theaccentuation vof the noise amplitude and the compression .of the noisespectrum into a narrow frequency band, which characterizes the systemsof the prior art. We have found that the random noise passed by anon-.oscillatory system is much less deleterious to the final image on akinescope, for example, than the periodic noise of prior art systems.The objectionable noise phenomenon of the prior art systems occurs eventhough the damping factor is extremely high. Circuit Qs of even 1, 2 or3 show the described effects, and it becomes worse as the number ofstages is increased, particularly if each stage is tuned or exhibitsperiodicity effects close to the proceeding stage, since then each stageacts as force-driven generator for those particular frequencies. If theQ of the system is raised, then the noise multiplication is alsoincreased. Therefore, tuned circuits that select frequencies, as. forexample, the tuned circuits of radio. receivers or television receivers,are particularly prone to, the .objectionablenoisephenomenon, whateverthe source of the noise. 1t isevident that if asystem is employed whichis not at least critically damped as a whole, it is very advantageous-to employ in the inputsections of the system amplifiers `which are atleast critically damped so that a vrelatively high level low noisesignal will be available for appli cation to the remanderof the system.Thus, the use of such an amplifier as the first stage or booster for aconventional television system or other conventional communicationsystem would greatly improve the performance of the latter.

The use of circuits which are at least critically .damped toprevent theaccentuation of noise is not limited to the particular system disclosedin Figure l. This method of signal translation `may b e applied to themeasurement .of potential existing in vmuscles of the human body. forexample, o r to the measurement of potentials occurring in ythe brainas'measured on an electro-encephalograph. lun the past, `very-narrowjfrequencyjands have beenused ip Asuch equipment just to keep the-noise level extremely low. However, these frequency bands could beincreased in width, with accompanying increase in resolution, if theprinciples of the invention set forth above were applied. The principleso f the invention may be applied to telemetering oircuitsrequiring Wide.band Spectrums or highly discriminating tuned circuits, or to systemsfor analyzing frequency or wave shape, where signal-to-noise ratios arelow `and Where highly discriminating periodic circuits are employedtoseparate frequency components. In the field o f color as -well asmonochrome television, practice of the principles of theinvention willallowthe use of considerably lower light levels with accompanyingadvantages of economy and the reduction of the effects of strong lightson the actors. In general, .the principles ofthe nventionmay be appliedto any system for gathering, detecting, transducing, reproducing, ortranslating, intelligence or signals wherein noise, interference or thelike existsat a level approaching the level of -the intelligence orsignals, and in particular where large gains or amplification arerequired. Where associated equipment must be .introduced to compensate`for phase effects, all these circuits must be kept at least criticallydamped in order to obtain the best reproductions.

It will lbe vappreciated that the invention is not limited to the use-of line amplifiers or the like, since other amplifiers may beconstructed 4having at least critically damped characteristics.Moreover, the principles of the invention maybe applied to transistorcircuits as well as vacuum tube circuits. The scope of the invention isnot limited to the foregoing embodiments, and such embodiments should,therefore, be taken as exemplary of 'the principles of the inventionrather than as restrictive. The'bounds of the invention are set forth inthe following claims.

We claim;

-1. In combination, a` source of invisible radiation, means forconverting radiation from said source to visible light having a lowVlevel of intensity of the order of l0'-2 millilamberts or less, saidapparatus comprising light responsive pick-up means for picking up andconverting said low-level light to corresponding electrical signals inthe presence of electrical noise having a level of intensity of the sameorder of magnitude as the intensity of said signals, amplifier means foramplifying said signals, means coupling said amplifier means and saidpick-up means, reproducing means for converting said amplified signalsto reproduce said visible light, means coupling and reproducing meansand said amplifier means, and electrical damping means for at leastcritically damping all the modes of oscillation of said pick-up means,said amplifier means, said reproducing means, and both said couplingmeans throughout their range of operating frequencies so that thesignal-to-noise amplitude ratio is not .decreased and the randomness ofthe noise is preserved..

2. Apparatus for intensifying light having a low level of intensity ofthe order of "2 millilamberts or less, said apparatus comprisinglight-responsive pick-up means for picking up and converting saidlow-level light to corresponding electrical signals in the presence ofelectrical noise having a level of intensity of the same order ofmagnitude as the intensity of said signals, amplifier means foramplifying said signals, means coupling said pick-up means and saidamplifier means, reproducing means for converting said amplified signalsto reproduce said visible light, means coupling said reproducing meansand said amplifier means, and electrical damping means for at leastcritically damping all the modes of oscillation of said pick-up means,said amplifier means, said reproducing means, and both said couplingmeans throughlout their useful frequency range so that thesignal-tonoise ratio is not decreased and the randomness of the noise ispreserved.

3. A system for producing an intensified lluoroscopic image, comprisinga source of X-radiation, means for projecting radiation from said sourcethrough a subject and onto a fluorescent screen to produce an image onsaid screen having a lower level of intensity of the order of 10-Zmillilamberts or less, means for picking up said image of low levelintensity and for converting said image to corresponding electricalsignals in the presence of electrical noise having a level of intensityof the same order of magnitude as the intensity of said signals,amplifier means for amplifying said signals, means coupling saidamplifier means and said pick-up means, means for reproducing said imagefrom said amplified signals, means coupling said reproducing means andsaid amplifier means and electrical damping means for at leastcritically damping all the modes of oscillation of said pick-up means,said amplifier means, said reproducing means,

and both said coupling means throughout their useful frequency range sothat the signal-to-noise ratio is not decreased and the randomness ofthe noise is preserved.

4. Apparatus for translating low level signals in the presence ofelectrical noise of the same order of magnitude, comprising pick-upmeans for picking up said signals, amplifier means for amplifying saidsignals, coupling means for connecting said pick-up means to saidamplifier means, indicating means for indicating the amplified signals,coupling means for connecting said amplifier means to said indicatingmeans, and electrical damping means for at least critically damping allthe modes of oscillation of said pick-up means, said amplilier means,said indicating means, and both said coupling means throughout theiruseful frequency range so that the signal-to-noise ratio is notdecreased and the randomness of the noise is preserved.

5. The apparatus of claim 4, wherein said amplifier means comprises aline amplifier.

References Cited in the file of this patent UNITED STATES PATENTS2,234,806 Ploke Mar. 11, 1941 2,319,712 Williams May 18, 1943 2,422,287Edwards May 25, 1948 2,555,424 Sheldon June 5, 1951 2,559,515 PourciauJuly 3, 1951 2,637,786 Bordewieck May 5, 1953 2,670,408 Kelley Feb. 23,1954 OTHER REFERENCES Amplifying and Intensifying the Fluoroscopic Imageby Means of a Scanning X-Ray Tube, Robert J. Moon, Science, vol. 112,October 6, 1950, pages 389-395.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.2,899,494 August ll, 1959 Ralph E. Sturm et al.

It is herebjr certified that error appears in the -printed specificationof the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.

Column l, line 30, for "better that" Iread better than column 4, line'5, the Word "necessary" should appear italized; column l2, lineA 67,claim l, for "coupling and" read coupling said column 13, line 23, for"lower" read low Signed and sealed this 15th day of March 1960.

(SEAL) Attest:

KARL Ilo AXLINE ROBERT C. WATSON Commissioner of Patents Attesting OHcer

